Conventionally, photocationic polymerizable adhesives containing an epoxy resin as a main component have been used as one kind of adhesive used when mounting an electronic part, such as an IC chip, on a circuit board. A photocationic polymerization initiator, in which protons are generated by light to cause cationic polymerization to start, is blended into such a photocationic polymerizable adhesive. Sulfonium antimonate complexes are known as such a photocationic polymerization initiator.
However, sulfonium antimonate complexes have as a counter anion SbF6−, in which fluorine atoms are bonded to the metal antimony, so that fluorine ions are produced in large amounts during cationic polymerization. This causes problems with corrosion of the metal wiring and connection pads. Thus, the use of a sulfonium borate complex as a cationic polymerization initiator which uses instead of SbF6− a tetrakis (pentafluorophenyl)borate anion [(C6F5)4B−], in which the fluorine atoms are bonded to a carbon atom, has been proposed (Patent Document 1). In fact, the complex [p-hydroxyphenyl-benzyl-methyl sulfonium tetrakis(pentafluorophenyl)borate] of the following formula (1c) is commercially available.

However, when mounting an electronic part on a circuit board, in many cases the joining portion cannot be irradiated with light. Thus, attempts are being made to convert the specific sulfonium borate complex disclosed in the examples of Patent Document 1 into a thermal cationic polymerization initiator for a thermal cationic polymerizable adhesive. In such a case, not only is it required to reduce the amount of fluorine ions produced during cationic polymerization and improve the electrochemical corrosion resistance of the thermal cationic polymerizable adhesive, but in order to improve productivity, the low-temperature, rapid curing properties of the thermal cationic polymerizable adhesive also need to be improved.
[Patent Document 1] Japanese Patent Application Laid-open No. Hei 9-176112